This invention relates to trackers for weapon systems. More particularly, this invention relates to electro-optical (E/O) trackers.
Electro-optical trackers are useful in pointing weapons incorporated in air defense systems. Typically, these trackers operate on the video output of an electro-optical sensor, such as a visible light camera or a forward-looking infrared sensor, and estimate the location, within the sensor field of view, of the tracked target. Two error signals, proportional to the angular offsets between the target and the sensor line of sight, are provided at the output of the tracker. These error signals may be used in a closed servo loop to drive the sensor pedestal axes so as to keep the sensor pointed at the target.
Most current high-performance trackers utilize a correlation algorithm to perform the tracking function. Correlation algorithms are well suited for high signal-to-noise environments. The signal-to-noise ratio (SNR) may be defined as the peak image intensity divided by the root mean square (rms) background noise level. Trackers using a correlation algorithm suffer significant performance degradation at SNR's of two or below. This performance barrier seriously limits the usefulness of the tracker in adverse weather conditions or under battlefield-induced conditions that include smoke and dust. Additionally, correlation trackers generally cannot "coast" through periods of target obscuration, such as when the target passes behind a cloud, as the tracker will tend to "lock up" on the obscuration. The tracker will also have a tendency to fail if the target flies in front of a highly cluttered background with significant spatial correlation.
An alternative to the correlation tracker has been proposed by Dr. P. S. Maybeck, as described in his recent papers. P. S. Maybeck and D. E. Mercier, "A Target Tracker Using Spatially Distributed Infrared Measurements," IEEE Transactions on Automatic Control, Vol. AC-25, pp. 222-225, April, 1980; P. S. Maybeck, R. L. Jensen and D. A. Harnly, "An Adaptive Kalman Filter For Target Image Tracking," IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-17, pp. 173-179, March, 1981. Maybeck's tracker employs a Kalman filter instead of a correlation algorithm. Through simulation, Maybeck has achieved a consistent order of magnitude performance improvement over that of existing correlation trackers.
A major shortcoming of the Maybeck tracker arises from the fact that only two-dimensional information is available from the E/O sensor. Due to the motion of the sensor line of sight which occurs when tracking the target, a constant velocity target will have a changing velocity projection in the image plane. Modeling of the motion of the target in two dimensions under such conditions causes artificial dynamics to be introduced into the filter. Further, changes in image size as the target distance varies and changes in the image shape resulting from target aspect angle variation cannot be predicted with a two-dimensional model. Because Maybeck's apparatus is by definition a passive system, it is not possible to incorporate a third dimension.